Filter arrangement



March 8, 1932. MCCURDY 1,848,221

FILTER ARRANGEMENT Filed May 29, 1928 Ewyuerwy and 15} ATTORNEY Patented Mar. 8, 19321 UNITED STATES rnonn AND rnLneRArHooMPANmA coR-ronA'rIon 01? NEW YORK FILTER nnnanqnmnn r Application filed my 29,

'ture" of a network interconnecting an input circuit andranfoutput circuit, in which an inductive element, or reactor, is located in series between sa d mp'utand output C1I" CL iljJS and series circuits. including nductive and capacitative, reactances are' located shunt across said input and output circuits, the

' seriesand shunt elements being so propor tioned'that' the network will suppress sinusoidall current of a particular frequency, or

predetermined bandor range flowing the input circuits.

This invention has important applications in systems for rectifying alternating cursuppressed and eliminated, while the direct rent, and particularly in such systems employing rectifiers which produce a plurality of harmonic frequencies of the current sup plied thereto, these harmonics interfering with the quality of speech which may be transmitted over an exposed telephone circuit. Bylmeans of the network comprising the filter of this invention, one or more of these harmonic frequencies produced by the rectifier,or rectifier's, may be substantially current may be freely transmitted.

This invention is illustrated in the accompanying drawings, in which Figure 1 repre- 4n sents a diagram illustrating the broad form of the invention from'which specific embodiments may be' derived Fig. 2 illustrates a modification of the arrangement shown in isand employing. a p r ty b recurrent sections, and Fig. 3 showsfa Qunve illussinusoidal currents within a specified and from' j f I *RALPH G. MccURDY, or nnennwoon, NEW JERSEY; AssIGNon 'r'oAMnnIoAN t rnnntrating the characteristic performance of the wave filter of this invention.

- Referring to F ig. 1, there is shown a filter inter-connecting an input circuit A and an output clrcult B. This filter consists ofone" series inductance, 1;. e., a series coil, L and two shunt j branchesgea'ch composed of an inductance and a capacity in series relations shlp, windings L and L and condensers 0 and C respectively, representing "these shunt branches. Coils L and L are coupled together and arranged so that there is mu- 1 tual inductance therebetween. Coils L ,";L and L may be considered to exhibit resistances whichmay be designated r r andr of these three circuits.

If the branch-including wifiding L. at

cuit B be tunedby adjusting the magnitude of the inductance ofwinding 'L or of" the .1 capacityv of condenser C to resonance at particular frequency intended for suppres-- condenser G which shunts l the output cirsion, a substantial current I of the frequen'cy of resonance will flow through this shunt branch. Since 13 designates the resistance in this shunt branch, the voltage E at the output'c'ircuit B at'the tunedfrequency will be 1 13. The current I in this shunt branch will lag behind the voltageE of the input circuitfby an angle. whose tangent is i Lie.

the condenser G which shunts theinput cir cuit A may be adjusted inits'inductive or ca- The branch including the pacitative magnitudes, o'r byvarying the co'ur plingbetween its winding L and the wind ing L of: the branch shuntinggtheoutput circuit B, so that a current I of the same frequency to which the branch shunting the output circuit B is resonant will flow therethrough and will lead the current I by 90. If M designates the coupling existing be tween windings L and L then the voltage M1 0) induced in the circuit including the winding L will lead the current I by 90, and the current I by 180, and this voltage will therefore be 180 out of phase with respect to the voltage drop across the circuit including winding L. It will be apparent that by suitably adjusting the coupling M between the windings L and L the voltage produced across the circuit including windln L and condenser C i. e., I 0 may be balanced out b a voltage of equal magnitude and of opposite polarity produced by the circuit including'winding L and condenser,

C Accordingly, the current of the resonant frequency will be negligibleand ineffective in the output'circuit B.

It may be readily shown that the voltage ratio of this network may be given by the expression Em Zi-ZZZ; (1)

It will be apparentthat the ratio E /E of Equation (1) will be zero if the numerator of theexpression at the right-hand side of the sign ofthe equation is zero. Thus, the ratio Eg/E will be zero when m-mZ Z Z 0. (2) Substituting for m, Z Z and Z ,Vthe values determined by Expressions (la), (16), (1c) and (1d), it will be clear that the following expression will be true: c M w jMw(7' +jL m 2+7' 2)( +i 3)= By the process of expansion, Equation (20) becomes I I M 0) jMwr +ML w r 9 j1' X jX 7' +X X =0 (25) Clearly, the real and'imaginary components of Expression (26) may be equated to zero, the real components giving the expression c [M, +ML Q +1- T +X ,=O 3 and the-imaginary components, the expres- Let the branch including winding L and condenser C be tuned to resonance at the frequency intended for suppression, then L w=1 0 a), and the reactance in this branch will be zero. At the frequency of resonance, since the reactance X is zero, Equation (3) becomes 7 and Z|1=kL (60) If the value of M determined by Expression (60) be substituted in Expression (6a), then Upon inspection of-Equat-ion (7), it will be apparent that there is one value of at which the inductance of winding L will. be a minimum. A low inductance for winding L is particularly desirable because in power circuit rectifying systems a heavy current will flow over the main line and therefore through I winding L. A low inductance will have but a few turns, and its cost will not be prohibitive.

Upon differentiating Expression (7) with respect to la, and equating the result to zero, the following condition will hold:

IC LQQ l Lg 0 I Since windings L and L were assumed to have equal inductances, assume also their re sistances 1 and a (or the resistances of the branches including these windings) to be equal, and Expression (7a) becomes i in which Q represents the ratio of reactance to resistance, or wL /T .isseaam I o lfl swii l e aminii m when-the coefli dent-o coupl ngi AQZ- :zA-Xitll QSKIQWHS m y becQnYenien -ly as gned o Qainipr e tisi iThe z ;1Wl ,,-Q i: acouplingewhieh inmat eae lyza tein hl :withiairflcorecoils. *A s0',-;theshun b a chesiincludingarind g L2 and Lrmayrbe tm deup; ofzsmall compact coils since a largeyalue f0DQ -iS3I10tsneGeS- sary, and consequently; theirrcost. willibe comgparatively small.

Since Zc=1/Q =1- /L e, then if this value of, H besuhstituted iorzthelcrinEquation .(7)

in-mute .foundzthat will be apparent that-theconstants: of a; were filter; fonthesuppression of? a particular :fre-

q en y may be readilydetermine PO fth assumption howevery that the inductances, of windings L i and L iand the resistances inseriestherewith are equal. -A- somewhat difl'erentv -.r elationshipwilL exist when the inductances of@windings L :and- Lg are unequal. Upontransposing the first and third terms of Equation 5) the llowingexpression will result: q a

To @p'roduce.v a Jninimumr-of inductance :aiinding L azwithr respect? .to :the mutual ind ct-ancelfMgit is necessary to difl'erentiate llquationgq(l0) with respectito M1 and equatethe result'to; zero. I 7 Then audit will belclear from. Expression (612)- t I I v v s wea i r Substituting; in iEquation" (19 the syalne; of

* M determined by Expression (10b) then;

lawilhnomzbe clear; thaeflm constants of the;- filter fonthe, suppression of a particular figqllencl m yw b e s y .1 determined; even when theg nluct nceslo zwind ngs L'ziandfLa arennequal. in this; case-, -.the-ratio]of ,reactf -to r s s a h a o i -th induc ance ctiwindigg I to rtha otlwil d l Lean th sllqpation; (8a):shewsthanz heinductanc lt ofnthe:alie isllances'inS rieSQWi itheSe inductances-imay be setat anydesired Values. I In case; aifilterlis required which shall be suitable fort-the elimination of several frely mayj. be employed in association withla acommonjseries coil or reactor such as is designated L Manifestly, vthis may be done because, the ,shunt elements: are composed of series resonant circuits in parallel relationship so; arranged that the resonance; adjustment of one will not .be efiected bythe preszence, oftheiother shunt branches.

I tEigfiishows twoof a plurality of sections which may be interposed betweenthe input circuit "A; and the. output, circuit B; these sections includingseries reactiveelements L and L respectively. ,1 Such an arrangement J3 l I ,rquencies, OIZOL several bandslof frequencies-2r J 0 number ofashunt elements composed of in- :ductanceandcapacity such asthose desigsnatedjL andC and L andaG respectiveishighly desirable for thelelimination of two i or more particular frequencies, orgof two or -morenarrow, bands of frequencies, andit will-beclearly: understood that other sections similar to those shown maybeconnected between circuits A and; B withinthe'scope of 'thB lDYQHtIOII. I I

In Fig. 2, the circuit ineludingxvinding L;

and condenser C shuntstheiinput,circuit A; I I --As;,-,1n% F1g. 1 of-the draw1ngs,-a=;c1rcu1t'1n- I .cludingwind ng L and condenser C5 is, coupled-thereto. The circuits including wind- .ing:- L and condenseryCi, winding L and condenser C may be similar in all respects to {the circuits including -windingL and condenser-C2, and winding L3 and condenser C3, respectively. ,The circuit ncluding windingLi and condenserC 1s tunedto be resonantat, a differentf-requency, or at a dife fe-rent band of; frequencies,, from the, frequency or band-of frequencies to which the circuit including winding L and condenser C is tuned. The two sections shown, which include; the series'reactors, L and :L ref spectwely, are associated withfcircuitsrwhlch need not have identical or substantially equal components of reactance and resistance. It will be'clearly understood that any number of other tuned circuit-s, similar to those shown,-may be employed in connection with each section these clrcu ts belng suitably coupled together-as shown and employed for the quencies or bands of frequencies; V It will be apparent that the elements of the circuits associated with-reactor Lia're idepurpose of suppressing other particular h I ca signated by primed reference characters and I that these may be similar to those associated with reactor- L It will also be apparent that I I each pair of'series circuits may be adjusted intheir coupling toJproduce equality of the V voltages oi a particular frequency, or band of frequencies, to be-suppressed and; that phase adjustment-maybe producedby Variations, in I i the magnitude of the inductance or of the ca acity, or both, included therein.

ig. 3 of the drawings shows a curve characteristic of the network shown herein, this curve exhibiting the relationship between frequency and the ratio of the voltage at the output circuit B to that at the input circuit A. This curve has been constructed from the constants determined by a network which may suppress current having a frequency such, for example, as 720 cycles.

The curve shows that the voltage ratio is zero at the particular frequency f to be eliminated, and that the voltage ratio is substantially negligible at a band or range of frequencies about point f. However, the curve shows a high voltage ratio at a frequency substantially different from and lower than frequency J, and yet since the voltage ratio is greatly ifi'erent from a negligible amount, this condition seems insignificant.

While this invention has been pointed out in certain particular arrangements, merely for the purpose of illustration, it will be understood that the general principles of the invention may be applied to other and widely varied organization without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A filter comprising a series circuit having an element of inductance and two shunt circuits each including an element of inductance and an element of capacitance, theinductances of said shunt circuits being'mutually reactive, the elements of inductance and capacitance of one of said shunt circuits being tuned to a frequency or to a narrow band of frequencies, to be substantially eliminated, said filter eliminating current of said frequency or currents within said narrow band of frequencies impressed thereon.

I 2. A filter comprising an inductance as a series element'and first and second'series resonant circuits as shunt elements, the inductances of said series resonant circuits being mutually reactive, the first of said series resonant circuits being tuned to the frequency of the current to beeliminated, and means associated with the second of said series reson ant circuits whereby there may be produced in the first of said series resonant circuits a voltage at the frequency of the current to be second series circuits, the first series circuit being tuned to the frequency of the current or to the frequencies of the currents within the narrow band to be substantially suppressed, and means whereby the second series circuit may produce in the first series circuit a voltage at the current of said frequency or at the currents within the narrow band of frequencies to be substantially suppressed equal and opposite to the corresponding volt age or voltages produced by the first series circuit.

4. A filter interconnecting an input circuit and an output circuit, comprising a series inductive element, two mutually reactive shunt elements which shunt the input and output circuits respectively, and means for rendering the element in shunt with the output circuit resonant at the frequency ofthe current to be eliminated so that the voltage at the frequency to be eliminated impressed thereacross may be brought to a predetermined value, said elementsbeing so related that the voltage at the frequency to be eliminated which becomes impressed across the element in shunt with the output circuitwill be balanced out by another voltage producedby said elements which is equal in magnitude and opposite in phase to that produced across the element in shunt with the output circuit.

5. A filter comprising, in combination, an input'circuit, an out ut circuit, a winding of low inductance serially connected between said input and said output circuits, a first series circuit including inductance and capacity in shunt with said input circuit, a second series circuit including inductance and capacity in shunt with said output circuit and resonant at the frequency of the current to be suppressed, and means whereby said first series circuit may produce in said second series circuit a voltage at the frequency of the current to be suppressed substantially equal to that produced by said second series circuit at the same frequency, both of said series circuits being so arranged that the relative phases of the voltages of the resonant frequency impressed upon said second series circuit may be brought to opposition.

6. The combination with an input circuit and an output circuit of a filtering arrangement comprising a reactor serially placed between said input and outputcircuits, four series circuits each having an inductive reactance and a capacitive react'ance, two of said series circuits shunting the input circuit, the other two of said series circuits shunting the output circuit, each series circuit shunting the output'circuit being'resonant at the frequency of one of the currents to be suppressed, and means including said series circuits shunting said input circuit to establish in the two series circuits shunting said output circuit voltages at the frequencies of the currents to be suppressed substantially equal and opposite to those voltages produced at the same frequencies respectively by the series circuits shunting the output circuit;

7. A filter interconnecting an input circuit and an output circuit, comprising ,a reactor in series between said input and output circults, a series resonant clrcuit 1n shunt w1th said output circuit freely b-y-passing current to be suppressed, a series circuit in shunt with said input circuit not resonant with the series circuit shunting said output circuit, and means to couple said series circuits so that the voltage of the current intended for suppression hy-passed by the series circuit shunting the output circuit may be neutralized by a voltage equal in magnitude aud opposite in phase to the voltage of the current of the same frequency by-passed by the series circuit shunting the output circuit.

In testimony whereof, I have signed my name to this specification this 26th day of I May, 1928. I

RALPH e. MOOURDY. 

